Electronic amplifiers having favorable forward transimpedance characteristics are very useful in numerous fields including audio, video, communications, computers and control systems. An ideal transimpedance amplifier would transform an input current to an output voltage by a given ratio. An ideal transimpedance amplifier would have infinite frequency response, slew rate, voltage gain, current gain, and forward transconductance; zero input impedance, output impedance, input bias current, and distortion; unlimited available output current; and transimpedance characteristics which are determined only by a feedback impedance. Unfortunately, all real amplifier designs fall short of ideal characteristics to one degree or another, and such shortcomings affect the suitability of particular amplifier designs for particular applications.
Typical prior art transimpedance amplifiers use various combinations of bipolar transistors, connected in common emitter or emitter follower configurations, connected as a Darlington pair, or as a differential input amplifier stage. Depending upon the requirements for a particular application, such prior art circuits may suffice, or in some cases it may be possible to further improve them by adding additional stages and components, but at the disadvantage of added cost, complexity and in some cases distortion. In my prior U.S. Pat. application Ser. No. 905,841, now U.S. Pat. No. 4,801,893, I disclosed a series of improved transimpedance amplifiers using FET first stages followed by one or more bipolar stages. These amplifiers overcome most of the shortcomings of the above-noted prior art designs.
The present invention provides a different approach to transimpedance amplifier design, one that provides even further performance gains as compared to the above-noted designs, while maintaining the advantages of simplicity, ruggedness and low cost. The present invention makes use of a combination of a vacuum tube and one or more MOSFET devices, in a unique combination which takes advantage of important characteristics of both devices. Various combinations of vacuum tubes and semiconductor devices have been used before, but not in the configuration used in this invention. Those configurations are not appropriate for an improved transimpedance amplifier. See, for example, U.S. Pat. No. 4,163,198, which is a form of a series connection of a p-channel FET and a triode. This circuit differs from the present invention in that it does not invert phase, and consequently it cannot be used as a transimpedance amplifier.